The organic light emitting device is an electronic device which emits light through the current by an applied voltage. Tang et al. report an organic light emitting device having good characteristics [Applied Physics Letters 51, p. 913, 1987]. Further, an organic light emitting device using a polymeric material, which employs the structure of the organic light emitting device as disclosed in this document, has ever been developed.
The essential point of the prior art is that the organic material layers in the organic light emitting device play their own roles in the processes for light emission, i.e. charge injection, charge transport, exciton formation, and light generation, respectively. Therefore, in recent years, an organic light emitting device comprising an anode (7), a hole injecting layer (6), a hole transporting layer (5), a light emitting layer (4), an electron transporting layer (3), an electron injecting layer (2), and a cathode (1), as illustrated in FIG. 1, or other organic light emitting devices having a more complex structure comprising additional layers is used.
Studies on doping various materials for improving the conductivity of the organic material for a hole injecting layer, a hole transporting layer, an electron transporting layer, and an electron injecting layer in the organic light emitting device have been conducted. See, Japanese Patent Application Publication No. 2000-196140, [Applied Physics Letters, 73, p. 729-731 (1998)], [Applied Physics Letters, 72, pp. 2147-2149 (1998)], U.S. Pat. No. 5,093,698, and International Patent Application Publication WO 01/67825.
The above documents proposed embodiments of devices having high efficiency simply by increasing the conductivity of the charge transporting layer or the charge injecting layer through doping. For example, International Patent Application Publication WO 01/67825 describes that the hole conductivity in the case of p-doping with a stable organic molecular material of an acceptor type having a high molecular weight of 200 g/mol or more on a hole transporting layer (at a low doping concentration of 1:110 to 10000) is further increased than those in the case of not applying such the procedure. Similarly, it is also described that by n-doping with a stable organic molecular material of a donor type having a high molecular weight on an electron transporting layer, the similar results can be obtained.
Meanwhile, in the currently available organic light emitting devices, the efficiency of the devices may have been increased by reducing the conductivity of the hole transporting layer because the degree of electron injection from the electron transporting layer to the light emitting layer is less than that of hole injection from the hole transporting layer to the light emitting layer [Applied Physics letters, 86, 203507, 2005].
However, this document describes that a hole injecting layer having a small energy band gap [copper phthalocyanine (CuPC), HOMO: −5.1 eV, LUMO: −3 eV] is doped on a hole transporting layer having a high energy band gap [N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB), HOMO: −5.5 eV, LUMO: −2.4 eV]. In these devices, the increase in the efficiency is resulted from increasing the efficiency in proportion to the ratio of the holes and the charges injected to a light emitting layer by trapping the holes using the HOMO (highest occupied molecular orbital) energy level of CuPC.